1. Field of the Invention
The present invention relates to a motor speed control circuit.
2. Description of the Related Art
Recently, heat generation from such an integrated circuit as CPU (Central Processing Circuit) used for electric equipment has been increasing as the operation speed of the integrated circuit increases. The CPU's increase in the amount of heat generated causes problems such as a thermal runaway. To deal with this problem, electric apparatus is usually provided with a fan for cooling the CPU.
FIG. 16 depicts an example of a block diagram of a motor speed control circuit that controls the rotation speed of a fan motor for cooling a CPU (see Japanese Patent Application Laid-Open Publication No. 2007-68344). Specifically, a speed control signal from a microcomputer etc., that corresponds to a target rotation speed of a motor 500 and a temperature signal from a thermistor that corresponds to a temperature around the CPU are input to a reference voltage circuit 700, and outputs a reference voltage corresponding to the speed control signal and the temperature signal. An FG (Frequency Generator) signal corresponding to the rotation speed of the motor 500 is input to a speed voltage output circuit 701, and outputs a speed voltage corresponding to the FG signal. A comparison circuit 702 compares the reference voltage with the speed voltage to output a drive signal as a comparison result. A motor driving circuit 703 drives the motor 500 based on the drive signal so that the speed voltage matches the reference voltage.
FIG. 17 depicts the relation of the rotation speed of the motor 500 in the motor speed control circuit 600 in FIG. 16, the speed control signal, and the temperature, which is obtained on the basis of an embodiment disclosed in Japanese Patent Application Laid-Open Publication No. 2007-68344. In FIG. 17, T1 to T3 denote temperatures that have a relation of T1<T2<T3. The speed control signal input to the motor speed control circuit 600 is a PWM (Pulse Wide Modulation) signal, and the rotation speed of the motor 500 increases according to the duty ratio of the H level (high level) of the PWM signal. The motor control circuit 600 feeds back the FG signal corresponding to the rotation speed of the motor 500 to generate the speed voltage, and compares the speed voltage with the reference voltage. As a result, the rotation speed of the motor changes linearly relative to the duty ratio of the H level of the PWM signal. Even when the duty ratio of the H level of the PWM signal is constant, the rotation speed of the motor increases as a temperature rises.
A recent fan motor, however, requires such operation that the rotation speed of the motor is changed linearly relative to the speed control signal and is changed according to a temperature, and that the rotation speed of the motor is minimized regardless of a temperature when the speed control signal is input so as to minimize the rotation speed of the motor. According to the motor speed control circuit 600 of FIG. 16, however, even when the PWM signal is input so as to minimize the rotation speed, the rotation speed of the motor 500 changes as a temperature changes, as shown in FIG. 17.